Problem solved with expensive "spin on" oil filters.
I found a common filter for Fords ($3), Volkswagens (same filter, but $5),
etc. of the same dimensions as my standard filter ($15). The threads were a
different size -- the Spitfire has a 5/8"-18 UNF thread while the
replacement has a 3/4"-16 UNF thread. So, I replaced the nipple(?) between
the block and the filter by machining a piece of hex bar stock to the same
dimensions as the original piece (with the bigger threads!).
All done! I only hope that my increase in oil pressure (80psi vs. 60 psi
from before) is due to replacing 10W30 oil with 20W50, and not because I
screwed something up with a different filter!
Thanks to everyone who replied to my original question.